Boron, lithium and methane isotope composition of hyperalkaline waters (Northern Apennines, Italy): terrestrial serpentinization or mixing with brine?
Résumé
Spring waters issuing from serpentinized ultramafic rocks of Taro-Ceno Valleys (Northern Apennine, Emilia-Romagna region, Italy) were analyzed for major, trace elements and dissolved gases concentration and isotopic composition of delta11B, delta7Li, delta18O(H2O), delta2H(H2O), delta13C(CH4) and delta2H(CH4). Similarly to other worldwide springs from serpentinites, the chemical composition of the waters evolves with water-rock interaction starting from Ca-bicarbonate, passing through Mg-bicarbonate up to an hyperalkaline Na-(Ca)-OH composition. Most of the Ca- and Mg-bicarbonate springs have a delta11B ranging between +16.3 and +23.7‰, consistent with the range of the low P-T serpentinites. Very high delta11B revealed in two springs from Mt. Prinzera (PR10: +39‰; PR01: +43‰) can be related to isotopic fractionations occurring during secondary phase precipitation, as also inferred from high delta7Li values. Differently from typical abiogenic signature of methane from serpentinited rocks, dissolved methane of Taro-Ceno hyperalkaline springs have an apparent biotic (thermogenic) signature with delta13C(CH4) ranging from -57.5‰ to -40.8‰, which is comparable with that of hydrocarbons from explorative wells and natural seeps in adjacent hydrocarbon systems. We may hypothesize that methane in the hyperalkaline springs investigated in this study may derive from organic matter in the sedimentary (flysch and arenaceous) formations occurring below the ophiolitic unit, whereas an abiotic serpentinization origin of methane is not evidenced, at the moment. Small amounts of H2 were however detected in the water and the occurrence of some abiotic CH4 cannot be excluded. Thermogenic CH4 in ophiolites may be a widespread phenomenon so that the identification of serpentinization-related gases needs accurate evaluations including a careful knowledge of the relationships with surrounding sedimentary rocks and their hydrocarbon potential.